Sorry, this one Bug()'d  really quickly. I applied this patch to
an unmodified test5 kernel.  Console output is attached.

On Tue, Sep 09, 2003 at 05:04:10PM +1000, Nick Piggin wrote:
> 
> Thanks Dave,
> Can you try this one? I can't reproduce the problem here as I don't have
> enough disks unfortunately. Thanks
> 
> Nick

>  linux-2.6-npiggin/drivers/block/as-iosched.c |   60 +++++++++++++++++++--------
>  1 files changed, 43 insertions(+), 17 deletions(-)
> 
> diff -puN drivers/block/as-iosched.c~as-warn-fix drivers/block/as-iosched.c
> --- linux-2.6/drivers/block/as-iosched.c~as-warn-fix	2003-09-09 17:02:41.000000000 +1000
> +++ linux-2.6-npiggin/drivers/block/as-iosched.c	2003-09-09 17:02:41.000000000 +1000
> @@ -373,7 +373,7 @@ static void as_move_to_dispatch(struct a
>   * direct IO, then move the alias to the dispatch list and then add the
>   * request.
>   */
> -static void as_add_arq_rb(struct as_data *ad, struct as_rq *arq)
> +static int as_add_arq_rb(struct as_data *ad, struct as_rq *arq)
>  {
>  	struct as_rq *alias;
>  	struct request *rq = arq->request;
> @@ -381,10 +381,20 @@ static void as_add_arq_rb(struct as_data
>  	arq->rb_key = rq_rb_key(rq);
>  
>  	/* This can be caused by direct IO */
> +#if 0
>  	while ((alias = __as_add_arq_rb(ad, arq)))
>  		as_move_to_dispatch(ad, alias);
> +#endif
> +	if ((alias = __as_add_arq_rb(ad, arq))) {
> +		arq->state = AS_RQ_NEW;
> +		list_add_tail(&arq->request->queuelist,
> +				&alias->request->queuelist);
> +
> +		return 1;
> +	}
>  
>  	rb_insert_color(&arq->rb_node, ARQ_RB_ROOT(ad, arq));
> +	return 0;
>  }
>  
>  static inline void as_del_arq_rb(struct as_data *ad, struct as_rq *arq)
> @@ -1131,7 +1141,21 @@ static void as_move_to_dispatch(struct a
>  	 * take it off the sort and fifo list, add to dispatch queue
>  	 */
>  	as_remove_queued_request(ad->q, arq->request);
> +
> +	for (;;) {
> +		struct list_head *alias = &arq->request->queuelist;
> +
> +		if (!list_empty(alias)) {
> +			struct request *rq;
> +			rq = list_entry_rq(alias->next);
> +			list_del(&rq->queuelist);
> +			list_add_tail(&rq->queuelist, ad->dispatch);
> +		} else
> +			break;
> +	}
> +
>  	list_add_tail(&arq->request->queuelist, ad->dispatch);
> +
>  	if (arq->io_context && arq->io_context->aic)
>  		atomic_inc(&arq->io_context->aic->nr_dispatched);
>  
> @@ -1303,22 +1327,22 @@ static void as_add_request(struct as_dat
>  		arq->is_sync = 0;
>  	data_dir = arq->is_sync;
>  
> -	arq->io_context = as_get_io_context();
> +	if (!as_add_arq_rb(ad, arq)) {
> +		arq->io_context = as_get_io_context();
>  
> -	if (arq->io_context) {
> -		atomic_inc(&arq->io_context->aic->nr_queued);
> -		as_update_iohist(arq->io_context->aic, arq->request);
> -	}
> -
> -	as_add_arq_rb(ad, arq);
> +		if (arq->io_context) {
> +			atomic_inc(&arq->io_context->aic->nr_queued);
> +			as_update_iohist(arq->io_context->aic, arq->request);
> +		}
>  
> -	/*
> -	 * set expire time (only used for reads) and add to fifo list
> -	 */
> -	arq->expires = jiffies + ad->fifo_expire[data_dir];
> -	list_add_tail(&arq->fifo, &ad->fifo_list[data_dir]);
> -	arq->state = AS_RQ_QUEUED;
> -	as_update_arq(ad, arq); /* keep state machine up to date */
> +		/*
> +		 * set expire time (only used for reads) and add to fifo list
> +		 */
> +		arq->expires = jiffies + ad->fifo_expire[data_dir];
> +		list_add_tail(&arq->fifo, &ad->fifo_list[data_dir]);
> +		arq->state = AS_RQ_QUEUED;
> +		as_update_arq(ad, arq); /* keep state machine up to date */
> +	}
>  }
>  
>  /*
> @@ -1495,7 +1519,8 @@ static void as_merged_request(request_qu
>  	 */
>  	if (rq_rb_key(req) != arq->rb_key) {
>  		as_del_arq_rb(ad, arq);
> -		as_add_arq_rb(ad, arq);
> +		if (as_add_arq_rb(ad, arq))
> +			WARN_ON(1);
>  		/*
>  		 * Note! At this stage of this and the next function, our next
>  		 * request may not be optimal - eg the request may have "grown"
> @@ -1526,7 +1551,8 @@ as_merged_requests(request_queue_t *q, s
>  
>  	if (rq_rb_key(req) != arq->rb_key) {
>  		as_del_arq_rb(ad, arq);
> -		as_add_arq_rb(ad, arq);
> +		if (as_add_arq_rb(ad, arq))
> +			WARN_ON(1);
>  	}
>  
>  	/*
> 
> _